Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 19. 2001. (Budapest, 2001)

Toarcian gastropods from the Gerecse Mts (Hungary) by András GALÁCZ & János SZABÓ Abstract — A relatively rich gastropod fauna has been found in the Toarcian of the Gerecse Mts, in a facies that is usually extremely poor in benthic fossils just like other localities of the Mediterranean Faunal Province. Nine species are separable in spite of the poor state of preservation. Most of them are deep water dwellers but two species seem "exotic" in the inferable palaeoenvironment. Keywords — gastropods, Toarcian, Gerecse Mts, Hungary. GALÁCZ, A. & SZABÓ, J. (2001): Toarcian gastropods from the Gerecse Mts (Hungary) — Fragmenta Palaeontologica Hungarica, 19: 15-24. Introduction The Toarcian marl sequence of the Gerecse Mts is a well-known source of fossils, mainly ammonites. Though only a few palaeontological papers were published, the ammonite stratigraphy of the Toarcian localides is well documented by the works of GÉCZY (1984, 1985). The only group besides the ammonites is the nautiloids, which got some attention in the past (PRINZ 1906). Collecting carried out during the past years evidenced that these two cephalopod groups represent almost exclusively the Toarcian marine macrofauna in the Gerecse, similarly to those other places where red marls have also developed. Under these circumstances it was rather a surprise to collect numerous gastropod specimens in a single locality of the Toarcian red marls. The study of these gastropods coming from Vöröshíd, north of Tardos in the eastern Gerecse revealed that the fauna is exceptional in richness and in diversity as well. Very rich and diverse gastropod faunas are wellknown from Sinemurian to Pliensbachian and from Bajocian deposits of many localities in the Transdanubian Central Range (see SZABÓ 1979, 1980, 1981, 1982, 1983, 1984, 1995, 1996), however, only a few Toarcian specimens has been found (SZABÓ 1982, 1995) so far. The assemblage from Vöröshíd is the first relatively varied Toarcian gastropod fauna. While the other faunas were found mainly in limestones of submarine topographic height origin, the Vöröshíd specimens occurred in "normal" Jurassic sediments of the wider area i.e. in marls deposited in basinal environment. These facts justify the separate report on these finds in spite of the rather poor (inner cast) state of their preservation. The specimens are deposited in the Geological and Palaeontological Department of the Hungarian Natural History Museum. Localities Toarcian red marls are widespread in the Gerecse Mts. Recendy a general description treated the distribution and the overall features of these sediments (CSÁSZÁR et al. 1998). Accordingly, this is a 0.5 to 6 m thick red, thinly-bedded, nodular claymarl, with variable carbonate and clay content, which constitutes the socalled Kisgerecse Marl Formation as lithostratigraphic unit. At its type locality, in the Kis-Gerecse Quarry, it is 3.5 m thick (KONDA 1986), and represents the lower and middle Toarcian, but the lowermost Toarcian Dactylioceras tenuicostatum Zone seems to be commonly missing (GÉCZY 1985). In some places (e.g. in the Tölgyhát Quarry near the northeastern margin of the mountains) a thin black shale layer is intercalated into the lower, Hildaites serpentinum Zone part of the sequence, while in other occurrences the corresponding beds are missing (JENKYNS et al. 1991). Well-preserved ammonites are frequent throughout, giving the chance to date all beds with high stratigraphie precision. Fragmenta Palaeontologica Hungarica 19, 2001 An evaluation of the thickness data from exposures and well logs made clear that there is a tendency of thickening of the Kisgerecse Marl towards the so-called Gorba High, a Jurassic submarine elevation forming now the western part of the Gerecse Mts (CSÁSZÁR étal. 1998). This phenomenon was interpreted as a res.lt of sediment winnowing from the submarine heights into the neighbouring, relatively deeper basin areas. The most important locality, Vöröshíd is an abandoned quarry on the eastern slope of the Kis-Tekehegy, near to the Tardos — Süttő road (Figure 1.). The road follows a significant fault-line, which now separates the Gorba Height on west, and the deeper, basinal facies area on east (LANTOS 1997). At Vöröshíd a series of quarries exposes the uppermost Triassic - Lower Jurassic sequence. KONDA (1987) and recendy LANTOS (1997) gave descriptions on the central, biggest quarry, which shows the most complete series. Here the deepest beds are the thick